This invention relates to a unit for positioning a sheet of film to cover the tops of product groups.
At present, stretch or shrink wrapping machines include units for positioning a sheet of film, cut to size, on the top of each group of products to be wrapped so as to provide a better hold on all the surfaces of the group of products.
These units may be used in any of several types of prior art wrapping machines and apparatuses, including, in a first solution, those with a vertically mobile ring where a roll mounting unit is moved relative to the product being wrapped.
In another solution, the wrapping apparatus comprises an L-shaped arm which mounts a roll unit that rotates around the group of products.
In yet another type of wrapping apparatus, it is the product to be wrapped that is moved relative to the roll unit.
Usually, a machine of the first type mentioned above comprises a mounting frame, normally bridge-shaped, and a roll mounting carriage supported on another structure which is in turn supported by the mounting frame. The mounting frame is normally located near a roller, belt or other suitable conveying system which feeds the products to be wrapped, usually on pallets, and positions them in the middle of the wrapping area.
The roll carriage mounts a roll of stretch plastic film and, usually, a unit for unwinding and pre-stretching the film from the roll.
The unit for positioning the top cover sheet may be positioned at the top of the mounting frame of these machines, said unit essentially comprising:
a roll of film positioned horizontally;
a power-driven gripper unit running on rails horizontally along the top of the wrapping area and designed to grip the free end of the film and to unwind a predetermined length of it, depending on the size of the group of products;
film cutting means located in the vicinity of the roll and designed to separate the length of film thus unwound from the rest of the film on the roll.
After being cut off, the sheet of film is released also by the gripper unit and allowed to fall onto the top of the group of products.
It is clear that a unit made in this way has several drawbacks due to: the low degree of precision in positioning the sheet; the high risk of the sheet getting crumpled as it moves down over the product; the low speed at which the operation is performed compared to usual wrapping machine speeds; and, in some cases, the incorrect distribution of the film on the top of the products (too much on some sides and too little on the others).
In one solution aimed at improving the units described above (see patent EP 336.517) a film unwinding unit and the gripper unit are moved simultaneously towards each other from opposite sides (by suitable means and guides) in such a way as to meet at the half way line of the wrapping area where the end of the film is gripped by the gripper unit and the two units are moved apart again so as to unwind the film to the required length which can then be cut off.
This solution, though an improvement over prior solutions, requires cumbersome guide structures, two drive systems and means for synchronizing the movements of the unwinding unit with those of the gripper unit, all of which add up to considerably increasing the size and cost of the unit.